The invention relates to the field of implants for use in a human body and, more particularly, to an implant having an integral attachment surface adapted to permit tissue ingrowth. A method of forming an attachment surface on an implant with a laser is included within the scope of the invention.
Many methods and devices have been developed to improve the fixation of an implant in the body so that the implant becomes as permanent as possible. One method well known in the area of orthopedic implants is the use of cements such as polymethylmethacrylate to anchor the implant. Another way to improve the permanence of implants is to construct the implants so as to receive an ingrowth of body tissue. For example, implants have been provided with porous surfaces as described in U.S. Pat. Nos. 3,605,123, 3,808,606 and 3,855,638.
U.S. Pat. No. 4,272,855 questions the merit of implants having porous surfaces implying that the mechanical strength of the implant is reduced by sharp corners and edges associated with such anchoring surface. According to this patent, these sharp corners and edges can lead to the formation of cracks, which may continue into the solid core of the implant and eventually lead to fatigue failures. It is disclosed that this disadvantage can be overcome with an anchoring surface which includes a plurality of depressions or projections devoid of corners and edges. The anchoring surface is described as produced by embossing, cast molding, chemical etching, milling or another mechanical kind of machining.
It is not believed that a surface devoid of corners and edges is best suited for tissue ingrowth. While a generally rounded surface does minimize the formation of stresses within the implant, it also minimizes the total surface area that can be contacted by the tissue. This reduction of surface area significantly reduces the strength of the attachment of the implant to the tissue, which is nearly totally dependent upon the mechanical interaction of the implant and the tissue. This mechanical interaction is generally of two types. One is a form of interlocking to the extent the tissue grows behind or around a part of the implant. The other is frictional, wherein the tissue grows into intimate approximation with the surface and results in a relatively tight frictional fit.
The anchoring surface of U.S. Pat. No. 4,272,855 minimizes the strength of both of these mechanical interactions. A totally rounded surface provides little structure into which the tissue can interlock. Furthermore, a totally rounded surface minimizes the area that can contact the tissue and frictionally restrict relative movement between the implant and the tissue. The implant of the present invention overcomes these disadvantages while still minimizing the creation of internal stresses within the implant due to sharp edges and corners.